1. Technical Field
The invention relates to a paging carrier assignment method for a multi-carrier system, and particularly, relates to a paging carrier assignment method for a paging group in a multi-carrier system and a multi-carrier base station and a wireless communication device using the same.
2. Related Art
A multi-carrier system has multiple carriers. If the multi-carrier system can not know on which carrier an advanced mobile station (AMS) camps, the multi-carrier system broadcasts identical paging messages on all carriers, i.e., paging messages are duplicated to all carriers. More specifically, this approach groups multiple carriers into one paging group, and an AMS can receive paging messages through any one of the carriers belong to the paging group. However, the paging message must be duplicated into multiple copies, for example, four copies duplicated for four paging carriers. Apparently, this approach does not take advantage of multi-carrier and there is no significant gain to paging capacity coming from multi-carrier.
In order to save radio resource, the multi-carrier system can configure an AMS receiving paging message on just one carrier and thus reduces paging message overhead. However, an AMS may move from a current serving cell to another cell. If the AMS can not know on which carrier the paging information is broadcasted after moving to another cell, the AMS shall send a location update message to the multi-carrier system so as to find out the selected paging carrier. If an AMS is in idle state, the AMS must perform network entry procedures for acquiring the selected paging carrier so that significant radio resource is consumed and this approach creates new problem and is uneconomical.
For example, a multi-carrier system assigns four carriers as one paging group but different AMSs may fetch paging messages on different carriers as illustrated in FIG. 1A. FIG. 1A is a schematic diagram illustrating an example of paging groups mapping for two advanced base stations (ABS). Referring to FIG. 1A, the multi-carrier system has at least two ABSs (labeled as BS1 and BS2). Within the coverage of the BS1, four carriers are grouped into one paging group, where BS1 uses carriers 1, 3, 5, 7, and BS2 uses carriers 2, 4, 6, 8. The four carriers (such as carrier 1, carrier 3, carrier 5 and carrier 7) are grouped into one paging group 1 in the MS1. Besides, a first AMS (i.e., MS1) just fetches paging message on carrier 1, a second AMS (i.e., MS2) just fetches paging message on carrier 3, a third AMS (i.e., MS3) just fetches paging message on carrier 5 and a fourth AMS (i.e., MS4) just fetches paging message on carrier 7. In this case, the message is not duplicated and the system can have more capacity to support more users in a paging system.
However, an AMS may move within the multi-carrier network as shown in FIG. 1B. FIG. 1B is a schematic diagram illustrating an AMS (such as MS shown in FIG. 1B) move among different advanced base stations (such as BS1, BS2, BS3 and BS4 in FIG. 1B) in a multi-carrier network 10. Referring to FIG. 1B, an AMS moves from the coverage of BS1 to the coverage of BS2, further to the coverage BS3 and to the coverage BS4 at last. According to the mapping shown in FIG. 1A, if the MS is the MS2, the paging carrier on which the MS fetches has to be changed from carrier 3 to carrier 8 when the MS is moved (i.e., handover or roam) from the BS1 to the BS2. However, the MS2 may not support the carrier 8, which is in a different band to the carrier 3. Thus, the MS2 may not camp on correct carrier and lose paging information when handover to the BS2. In such a situation, the MS2 must request the BS2 to re-assign a paging group or carrier to itself, and this carrier re-assignment process requires data updated in databases in the backhaul or the core network of the multi-carrier system, thereby consuming more system resources and time. Therefore, it is an important issue to find an effective and efficient carrier assignment scheme so as to lower the probability of carrier re-assignment process in a multi-carrier system.